This invention relates to a device for detecting an abnormal state of controls or of a plant, which is an object to be controlled by the former, in an automatic control system. More particularly, the invention relates to a diagnostic device for use with automatic control systems which is based on a model reference method wherein there is provided a model having a characteristic simulated to that of the controls or plant, and outputs of the model and plant are compared for detecting an abnormal state of the plant or the like.
A widespread use has been made of automatic control systems in industrial fields, and many of them are of a so-called closed loop type utilizing a feedback signal. More particularly, a plant, which is an object to be controlled, is operated by the output signal from controls while the controls are operated depending on the deviation between the plant output and the demand signal corresponding thereto. The output of plant is generally termed the feedback signal. In such automatic control systems, in the event of failure of the control system, there occurs a controlling out of the command of the demand signal originally intended and a consequent overrunning or running-down of the plant. In view of trouble shooting, the running-down of the plant has a tendency toward overall safety in general, and usually invites no serious problems. However, it can be disagreeable in a case where the plant controllable by the control system in question is connected with additional plants because the additional systems of plant may be adversely affected. The larger and more sophisticated the systems are, the more seriously they tend to be effected. Conversely, overrunning is often fatal, since it results in the possibility of serious damage of the component apparatus.
For the above reasons, in the past, it was the practice to quickly detect failures of the control system through various methods and take care of the plant suitably. One of the conventional methods for detection of the abnormal state of a control system is to monitor the deviation signal between the demand and feedback signals, as disclosed, for example, in Japanese Patent Publication No. 6815/72. This method is based on the fact that "the demand signal substantially coincides with the feedback signal under the stationary condition." Although having the capability of detecting the failure with high accuracy under the stationary condition, this method has many difficulties with the detection under the transient condition. For example, when the demand value is changed, the absolute value of the deviation signal increases. But this fact alone makes no distinction between a change of demand value (normal) and a failure of the system (abnormal). Additionally, with a retard response of the plant, the state remaining in increased deviation continues for a long time. Therefore, an increase in detection sensitivity of the diagnostic device is prone to an erroneous discrimination; conversely, for elimination of the erroneous discrimination, the detection sensitivity must be decreased. In any event, the deviation monitor method fails to detect the abnormal state under the transient condition.
Recently, in place of the aforementioned method, a method called the model reference method has been highlighted, as exemplified in a Japanese patent application laid open to the public as No. 58279/73, wherein there is provided a model having an input-output characteristic ##EQU1## for example, where G.sub.1 (S) represents an input-output characteristic of the controls and G.sub.2 (S) represents an input-output characteristic of the plant. The model is supplied with a deviation signal between the demand and feedback signals so that the output of the model is compared with the feedback signal representative of the output from the plant. The above W(S) is generally termed the total transfer function. In the model reference method, the model W(S) may be formulated in an analog or digital expression, and, especially, it is desirable to digitally formulate a sophisticated W(S) by means of a microcomputer or similar device. If there occurs no failure in the feedback control system whose abnormal state is to be detected, the signal passed through the model W(S) coincides with the feedback signal irrespective of any change in the demand signal, and the deviation signal between these two signals is always zero. Thus, in contrast to the aforementioned deviation monitor method in which the deviation signal varies under the transient condition even when the system is in a normal state, this model reference method is freed from variations in the deviation signal when the system is in the normal state so that detection sensitivity can be increased. At the same time, the erroneous discrimination is suppressed from occurring. Actually, however, this method, when reduced to practice, suffers from variations in the deviation signal for various causes even when no failure of an object to be diagnosed occurs, thereby giving rise to erroneous discrimination. Typical causes of this are:
I. Interference by noise PA0 II. Accuracy of the model
An object to be diagnosed standing for a plant or an analog automatic control system generally has a sophisticated characteristic expression. Consequently, the model is usually programmed for a digital operation device such as a computer instead of being constituted by an analog operation circuit. Thus, in order for an operation by the model programmed in a computer to be executed, it is necessary to convert analog outputs at component apparatus in the automatic control system into digital quantities. In other words, the analog output is sampled at a predetermined period .DELTA.t for its conversion into a digital signal of n bits. Digital signals, however, are sensitive to noises and the noises interfere with any bits of the digital signal at the same probability. No serious problems are raised if the noise interferes with bits of a lower digit, but the noise interfering with bits of an upper digit affects the model operation to a great extent, resulting in erroneous discrimination. Further, the actuation of various switches in the automatic control system causes direct spike noises to come into the analog signal, and it happens that the sampling is carried out in synchronism with the spike noise.
Practically, it is difficult to completely simulate a model to a characteristic of an object to be diagnosed. In particular, even if the model is formulated in a mathematical expression, there still remain difficulties with letting parameters in the mathematical expression coincide with an actually existing object to be diagnosed. Adjustment of such parameters will require characteristic tests on the object in many cases. However, complete characteristic tests are theoretically impossible, and therefore these tests should be terminated with a reasonable degree of incompletion. Since errors due to the parameters of the model are accumulated when the model has integrating terms, the deviation between outputs of the object and the model is increased with a lapse of time and erroneous detections of abnormal states result.